net/mlx4: Replace irq_to_desc() abuse

No driver has any business with the internals of an interrupt
descriptor. Storing a pointer to it just to use yet another helper at the
actual usage site to retrieve the affinity mask is creative at best. Just
because C does not allow encapsulation does not mean that the kernel has no
limits.

Retrieve a pointer to the affinity mask itself and use that. It's still
using an interface which is usually not for random drivers, but definitely
less hideous than the previous hack.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Tariq Toukan <tariqt@nvidia.com>
Link: https://lore.kernel.org/r/20201210194044.580936243@linutronix.de